You can download OS X Yosemite (10.10) for free from the Mac App Store if you purchased it in the past and it appears in your purchase history with the option to redownload the operating system. Once you download it, the installer will launch automatically. If you follow the onscreen instructions, you’ll end up with an upgrade installation of OS X (or macOS) Yosemite on your startup drive.
OSX Yosemite – How to Make a Bootable USB Stick
What if you want to perform a clean install, completely wiping your startup drive? Or maybe you want to have the installer on a bootable USB drive so you don’t have to re-download it every time you want to upgrade one of your Macs?
The problem is that you can’t upgrade another Mac without re-downloading the installer. The solution is to create a bootable USB drive with the OS X Yosemite installer.
There are two ways to create a bootable installer.
